Textual: We have specific words from Alma in the midst of description from Alma. We may presume that we are now reading Mormon's rewriting of Alma's record. While the only firm indication we have that Alma created a record is Mosiah 17:4 that specifically mentions Alma writing about Abinadi, we may safely presume that Alma also wrote a record of his efforts with those who followed him to Mormon.
Mormon's interest in Alma's writings begins with the words of Abinadi, both because of their effect upon Alma and their firm declaration of the essentiality of the atoning Messiah. When Mormon turns to the events at Mormon, he chiefly records events rather than specific discourses. This would be consistent with a record that chiefly recorded events rather than discourses.
We can speculate somewhat on the nature of Alma's record based on the way Mormon treats it. We know that Mormon is quite willing to record an original document such as we have with the copied record of Zeniff (beginning in Mosiah 9). We know that Mormon is willing to record a long but important discourse such as Abinadi before the priests. In the current text we also have some cited passages such as these that give the essential baptismal covenant Alma pronounced. What we also have is a large amount of descriptive text. This description rather than citation continues through the end of this chapter and includes a synopsis of Alma's post-baptismal sermon rather than a citation.
Why doesn't Mormon cite the full sermon? The topics appear to be important, and in similar instances, Mormon has allowed Alma and others to speak for themselves. It is possible that we are seeing an artifact of the source that Mormon worked with. It may well be that Alma did not record his words extensively, but that most of his record was narrative description. That is the type of material that Mormon could easily condense, and would produce precisely the type of text we have. Where Alma's record clearly included important copies of discourses, Mormon stays true to his editorial nature, and enters them into his abridgement. Where Alma simply told what he said, as opposed to writing down the text of what he said, Mormon again stays true to form and gives us a synopsis rather than a copy from the original source.
Historical: The simple explanation of this verse is very straightforward; the people respond enthusiastically to the offer of baptism. That simple answer, however, does not seem to fit all of the information we have presented to us.
Most interesting is the description of the people's reaction to the invitation to baptism. It is interesting not in that they want to be baptism, but that they clap their hands for joy. This reaction of the crowd invites speculation on what would cause the people to clap their hands. The incident is recorded as a collective action. As we saw in the discussion of Benjamin's speech, collective action can either be ritual or spontaneous. Which might this one be?
While it is possible that there was some cultural indication that one should clap at certain times, it really seems unlikely that Alma records, and Mormon writes, about an action that was an expected ritual event. It is much more likely that this is a spontaneous action, and is remarkable in its spontaneity and therefore worthy of note in both Alma and Mormon's records. We know that this clapping of hands for joy can be a very natural human reaction, and this strengthens our declaration that this was an unplanned public event.
Declaring that it is spontaneous is fine in and of itself, but the real question is why it was a spontaneous event. The clapping of hands is not only connected to joy, but to surprise. Little children at Christmas time might clap hands for joy when they see something that really excites them. When they see that same thing later, they may continue to be excited, they may continue to have joy, but the hand-clapping that accompanied the original instance is not repeated. The hand-clapping occurs at the original instance of something that is not only joyful, but unexpected.
In the case of the converts of Alma, what is unexpected? It is Alma's invitation to baptism. The converts in the land of Mormon do not expect that they will be baptized with they follow Alma. They great this offer not only with delight, but also with a spontaneous hand-clapping indicating that it is an unforeseen delight. As indicated above, the introduction of Christian baptism by Nephi would have supplanted the ritual washing of the mikveh. However, it appears that the rite of baptism may not have been widely practiced. Even presuming that Christian baptism would have been removed from Noah's religion, we cannot suppose that all who followed Alma were so young as to not have been alive during Zeniff's reign. During that time they apparently were not taught of a redemptive baptism, and therefore this opportunity to expunge their sins from their most recent life came as a most welcome opportunity. They were certainly convinced that they had been wrong, now they had the opportunity to clean the slate and start over again before God. That would have been a powerful offer.
They had followed Alma because they had come to understand the atoning Messiah who would come in some perhaps distant future. In spite of that far future event, Alma was offering them immediate access to the atonement that they had recently come to understand. They had given up all of their lives for their understanding of this atonement. Now they had the opportunity to participate in it directly and personally. No wonder the offer of baptism was not only surprising, but also an occasion of tremendous joy.